I mostly gel, but in the few batches I've done that did not gel, the resulting soap has always been as soft as cream cheese upon unmolding and quite crumbly if cut too soon. For this reason, it's best to wait 48 hours or so before unmolding and cutting it, or you can stick it in the freezer or fridge to firm up a bit first.
The size of your batch doesn't matter in terms of deciding whether to gel or not, or whether it turns out crumbly or not, although smaller batches may need a little bit more help to encourage gel along. Crumbling is normally formula-related or just a simple matter of having cut the soap at the wrong time.
How old is the soap that you made in your class? I ask because the older the ungelled soap is, the less apt it should be to be crumbly. It should be more cohesive and firm. Also- other things can cause crumbly soap, such as lye heaviness for example, or the addition of high amounts of certain additives such as sodium lactate or clay, etc..
[Oh, I just saw where you said the class measured in volume instead of weighing. Yeah, bad idea. I wouldn't take any more classes from them.]
Also- at the other end of the spectrum, if you wait
too long to cut certain formulas, the soap can crack and crumble from it simply being too hard to cut at that time.
Re: gel- Another way to explain gel is that it is just saponification speeded up. While all raw soap batters have to go through the process of saponification to become actual soap, the gel stage will just get them there that much quicker.
Gelling/not gelling is all a matter of personal preference. I'm one of those who likes to gel. I choose to gel because I like being able to unmold and cut quicker, thereby freeing my up molds to make more soap,
and because it cuts 2 weeks off my cure time (4 weeks instead of 6). I also like the transluscent look and how my colors turn out more brilliant instead of pastel.
And if I want an opaque/creamy or pastel look to my gelled soap, all I have to do is add a little TD to my batch and voila!- opaque, creamy and pastel looking soap. I can also get really cool opaque/translucent looks in the same soap by adding TD to only part of my soap. Gelled soap just gives me more options.
IrishLass